Food poisoning, lying on your stomach for too long, illness, etc. But
in any case, if you really want to make sure to avoid
pregnancy, you should use condoms in addition to birth control, that way if either one fails, you still have the other one. And if you want to make sure you're not pregnant, even if you don't think you are, take a home pregnancy test. They're about $7 at Meijers and you get results in three minutes.